The present invention relates to optical sensors used to measure changes in environmental parameters such as pressure, displacement, acceleration, and velocity. More specifically, the invention is directed to an optical pick-off as part of an interferometer that can be used in such a sensor.
Michelson or Mach-Zehnder interferometers have been known for use in certain applications, such as acoustic sensors. A description of a Mach-Zehnder interferometer used in an underwater acoustic sensor is contained in, for example, U.S. Pat. No. 5,448,058 to Arab-Sadeghabadi et al.
An optical interferometer of known type includes a pair of optical fibers into which a single source of light directs a light signal. The light signals, guided respectively through the two fibers, follow optical paths of different lengths, producing a phase difference between the two signal beams when the beams are combined. The combined beams may be detected by an optical detector. If the two signal beams have the same polarization state when they are combined, the signals interfere to form a fringe pattern of bright and dark lines that is detected by the optical detector.
Exposing either or both of the fibers to a change in the environmental parameters, such as an acoustic pressure change, changes the fringe pattern that is incident on the optical detector. Such changes in the fringe pattern as detected by the optical detector may be analyzed to measure the changes in the environmental parameters to which the fiber has been exposed. In this manner, the nature of the acoustic waves to which the fiber is exposed may be determined when the interferometer is used in an acoustic sensor.
Mach-Zehnder or Michelson interferometers employed in underwater acoustic sensor ("hydrophone") systems use tens of meters of optical fiber wrapped on a mandrel. The fiber is stretched and/or contracted to produce a measured phase delay that is proportional to the changes in pressure resulting from acoustic waves. The interferometer has an optical path length mismatch between its two optical legs that is on the order of one meter, to allow the standard functioning and signal processing with a phase-generated carrier. See, for example, Kersey, "Distributed and Multiplexed Fiber Optic Sensors", in Udd, Ed., Fiber Optic Sensors: An Introduction for Engineers and Scientists, (New York, 1991), pp. 347-363.
Fiber optic interferometric sensor systems, of the types described above, have found favor over piezoelectric hydrophone systems, due to such advantages as immunity to electromagnetic interference (EMI); the ability to locate all electronic and electrical components and systems in the towing vessel, rather than in the underwater environment; and enhanced capabilities for measuring vector quantities. The prior art fiber optic sensor systems, however, are relatively expensive to manufacture. Thus, less expensive alternatives that provide the same advantages over piezoelectric systems have been sought. Batch-processed silicon chip sensors, having a proof mass that is moved in response to changes in environmental parameters, such as pressure and acceleration (which may result from, for example, vehicle or medium motion), have been employed as accelerometers and velocity sensors. Such silicon sensors are very inexpensive and quite rugged. Use of such silicon sensors in a hydrophone system, with the proof mass accessed by a fiber optic delivery system, would lower costs as compared with prior art fiber optic systems. Making such chip sensors compatible with existing fiber optic architectures in Mach-Zehnder and Michelson interferometric sensing systems and the like has, however, proved troublesome in practice.
It would therefore be a significant advancement in the state of the art to provide a fiber optic interferometric sensor system, in a hydrophone or like application, that is capable of employing common, batch-processed silicon sensors.